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If errors are received when attempting to backup Outlook OST files in Window, this is likely because Windows Volume Shadow Copies do not include Outlook OST files (meaning that an OST file cannot be backed up while it is "in-use").
The following is Microsoft explanation of this design decision:
Maintaining changes to .ost files within shadow copies is expensive in terms of space and I/O activity. The performance impact doesn’t occur during the image backup itself--the only extra work at backup time is backing up the .ost file as part of the image. Instead, the performance impact occurs during the ongoing, everyday I/O to the .ost file when Outlook is running. If the .ost changes were kept in shadow copies, then every time Outlook writes to the .ost file, the result is a copy-on-write I/O hit (2 writes, 1 read). Although we have worked to reduce the impact of copy-on-writes on shadow copies, a heavily churned file like an .ost file could still cause problems. For these reasons, and the fact that .ost files can be regenerated, we chose to delete .ost files from the shadow copy before the image is created.
Even if the performance issues didn’t exist, there are situations where Exchange will, after an .ost is restored, detect a “future” version of the .ost file and force you to delete and then regenerate the local .ost file. Therefore, it’s still preferable to regenerate an .ost file instead of restoring it.
Because of this, OST files cannot be copied as in use files because they are not a component of VSS Volume Shadow Copies in Windows Vista and 7. As such, it is recommended that OST files NOT be included as part of a backup configuration due to this issue and also based on the fact that Exchange automatically regenerates OST files as needed.